Experts report that Eastern tent caterpillar eggs have begun hatching in Central Kentucky, just as leaf buds are swelling on wild cherry trees. While it is too early to tell what 2011 caterpillar numbers will be, populations have been increasing gradually over the past several years. Controlling Eastern tent caterpillars is vital to area breeding farms, as UK research results have strongly linked the caterpillars with Mare Reproductive Loss Syndrome (MRLS) outbreaks, which can cause late-term foal losses, early-term fetal losses, and weak foals.

According to Lee Townsend, PhD, University of Kentucky College of Agriculture entomologist, the newly hatched eggs, which caterpillars laid last June, are easy to identify by the small holes tiny larva chew as they exit. In about two weeks the "tents" should be baseball-sized and easy to spot in trees.

Five newly hatched eastern tent caterpillar eggs.

Townsend urged horse farm managers to inspect wild cherry and related trees for Eastern tent caterpillar activity to determine whether management is necessary. If control measures are needed to reduce numbers, implement them before the caterpillars leave their trees.

"The small caterpillars will stay near the egg mass for a short time before moving to feed on expanding leaves. Eggs will continue to hatch through early April," Townsend said.

"Eastern tent caterpillars grow and develop as long as the temperature is above 37 degrees; the warmer it is, the faster they will grow," he added. "Cold temperatures will slow them down, but the tent and the general cold hardiness of the species will keep them from being affected drastically, even if temperatures drop below freezing at night."

Townsend said it is too early to tell if 2011 will follow the recent trend of higher caterpillar populations, but stressed that population variability occurs with many insects. It is also normal for insects to be abundant in some parts of the county and moderate to low in numbers in others.

During the 2001-2002 MRLS outbreak, an estimated 30% of that year's Thoroughbred foal crop was lost. The state suffered an economic loss of approximately $336 million in all breeds of horses.

In the wake of the outbreak UK researchers conducted epidemiological and field studies that demonstrated that MRLS was associated with unprecedented Eastern tent caterpillar populations on Kentucky horse farms. Studies since the 2001-2002 outbreak subsequently have revealed that horses will eat the caterpillars inadvertently, and the caterpillar hairs then embed into the lining of the alimentary tract (which includes the pharynx, esophagus, stomach, small intestine, and colon). Once that protective barrier is breached, normal alimentary tract bacteria can gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death from these alimentary tract bacteria is the hallmark clinical sign of MRLS.

UK entomologists recommend that unless horse farm managers have been aggressive in managing Eastern tent caterpillars or removing host trees, they should keep pregnant mares out of pastures bordered by cherry trees or other hosts for the next several weeks.

Townsend offered the following recommendations for controlling moderate to large caterpillar populations if horses cannot be moved to avoid possible exposure:

"Foliar sprays for caterpillar control can be made when tents are about the size of a baseball," he said. "Another option is the injection of trees with a systemic insecticide by commercial pesticide applicators or arborists. Regardless of the treatment used, it is important to revisit the sites in about five days to assess caterpillar activity."

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